bug#41536: uniquify can select non-unique prefix

[ydirson@free.fr]

Package: emacs
Version: 26.3

After openning the following two files:

 /tmp/a/b/c
 /tmp/a/x/b/y/c

With style "forward" and all other customization vars as "standard",
the buffer names are respectively:

 b/c
 y/c

With my source-directory layout respecting the "higher-level is most significant"
principle, I would have expected the second buffer to be "x/b/c" instead.

In my case the "y" level is even a python package for modules containing abstract
classes, call it "lib" -- you'll understand that "lib/foo.py" is not really
helpful, when other packages could have a module of the same name in a "lib/"
subpackage.